I think there are two aspects. First, quite obvious, everyday life and overall infrastructure is really good. It is clean, well maintained in most places. And there are only few countries where all open landscapes are yours to have. Germany is one big park. You have a small garden? No problem, every forest or unfenced meadow is yours to use and to enjoy. The other aspect, which makes people leave, is the direction in which things are going. It feels like things get a little worse every day. Bureaucracy was never easy in Germany, but since Covid, it feels like certain parts of it have mostly stopped working. All that while they add more taxes, more regulation all the time. This is what drives motivated people out.
"And there are only few countries where all open landscapes are yours to have."
Well, I am into wild camping and here germany is definitely not so open. You have to be very well hidden.